The direct oxidation of ethylene to ethylene oxide by molecular oxygen is well-known and is, in fact, the method used currently for commercial production of ethylene oxide. The typical catalyst for such purpose contains metallic or ionic silver, optionally modified with various promoters and activators. Most such catalysts contain a porous, inert support or carrier such as alpha alumina upon which the silver and promoters are deposited. A review of the direct oxidation of ethylene in the presence of supported silver catalysts is provided by Sachtler et al. in Catalyst Reviews: Science and Engineering, 23 (1&2), 127-149 (1981).
It is also well-known, however, that the catalysts and reaction conditions which are best suited for ethylene oxide production do not give comparable results in the direct oxidation of higher olefins such as propylene. The discovery of processes capable of providing propylene oxide by vapor phase direct oxidation in higher yields than are presently attainable thus would be most desirable.
New support materials are continuously being tried. However, many of those which were employed in the early development of the silver-bearing catalysts are, with some modifications, still being used. Materials which have found most widespread use are typically inorganic and generally are of a mineral nature.
Alumina, in its various forms, particularly alpha-alumina, has been preferred as a support material for silver-containing catalysts in the preparation of epoxides. Numerous variations of surface area, pore dimensions, pore volume and particle size have been suggested as providing the ideal physical property or combination of properties for improving efficiency, activity or useful life of the catalyst.
In seeking the ideal support material, there has been some departure from the commonly employed substances. For example, some use has been made of alkali metal and alkaline earth metal carbonates, both as the sole support material and in combination with other materials as the carrier for processes such as direct oxidation of alkenes to epoxides. For example, Canadian Pat. No. 1,282,772 teaches the use of alkaline earth metal carbonates as supports for silver catalysts in olefin epoxidation systems.
The development of alternative supports which provide equivalent or improved performance in epoxidation process as compared to known materials would be highly advantageous, as such alternative supports may be of lower cost or provide other practical benefits such as higher strength or structural integrity. Selecting materials which will be suitable for such purpose is not straightforward, however. For example, as will be subsequently demonstrated, not all alkaline earth metal-containing compounds perform equivalently as supports for silver epoxidation catalysts. Structurally analogous substances often exhibit radically different behavior in an epoxidation process. Predicting in advance which substances will provide the high degree of selectivity to epoxide which is required in a commercial process thus is nearly impossible.
European Pat. No. 393,785 teaches a catalyst for the manufacture of alkylene oxide containing an impregnated silver metal on an inert refractory solid support, at least one promoter to enhance the efficiency of the catalyst and a manganese component. The efficiency promoter may be a compound comprising at least one alkali metal or oxyanion of an element other than manganese or oxygen selected from group 3b through 7b and 3a through 7a of the Periodic Table; titanates and phosphates are listed as being suitable oxyanions for such purpose. A maximum of 2 weight % of the anion in the catalyst is taught. A cationic promoter such as an alkaline earth metal may also be present up to a concentration of 1 weight percent in the finished catalyst. This publication thus does not contemplate the use of alkaline earth metal titanates or phosphates as the inert refractory solid support.